A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. The neural-functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions, and various areas of the brain appear to have distinct functions in most individuals. In the majority of people, for example, the occipital lobes relate to vision, the left interior frontal lobes relate to language, and the cerebral cortex appears to be involved with conscious awareness, memory, and intellect.
Many problems or abnormalities can be caused by damage, disease and/or disorders in the brain. Effectively treating such abnormalities may be very difficult. For example, a stroke is a common condition that damages the brain. Strokes are generally caused by emboli (e.g., obstruction of a vessel), hemorrhages (e.g., rupture of a vessel), or thrombi (e.g., clotting) in the vascular system of a specific region of the brain. Such events generally result in a loss or impairment of a neural function (e.g., neural functions related to facial muscles, limbs, speech, etc.). Stroke patients are typically treated using various forms of physical therapy to rehabilitate the loss of function of a limb or another affected body part. Stroke patients may also be treated using physical therapy plus an adjunctive therapy such as amphetamine treatment. For most patients, however, such treatments are minimally effective and little can be done to improve the function of an affected body part beyond the recovery that occurs naturally without intervention.
Neurological problems or abnormalities are often related to electrical and/or chemical activity in the brain. Neural activity is governed by electrical impulses or “action potentials” generated in neurons and propagated along synaptically connected neurons. When a neuron is in a quiescent state, it is polarized negatively and exhibits a resting membrane potential typically between −70 and −60 mV. Through chemical connections known as synapses, any given neuron receives excitatory and inhibitory input signals or stimuli from other neurons. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires a series of action potentials when the integration exceeds a threshold potential. A neural firing threshold, for example, may be approximately −55 mV.
It follows that neural activity in the brain can be influenced by electrical energy supplied from an external source such as a waveform generator. Various neural functions can be promoted or disrupted by applying an electrical current to the cortex or other region of the brain. As a result, researchers have attempted to treat physical damage, disease and disorders in the brain using electrical or magnetic stimulation signals to control or affect brain functions.
Transcranial electrical stimulation is one such approach that involves placing an electrode on the exterior of the scalp and delivering an electrical current to the brain through the scalp and skull. Another treatment approach, transcranial magnetic stimulation, involves producing a high-powered magnetic field adjacent to the exterior of the scalp over an area of the cortex. Yet another treatment approach involves direct electrical stimulation of neural tissue using implanted electrodes.
The neural stimulation signals used by these approaches may comprise a series of electrical or magnetic pulses directed toward affecting neurons within a target neural population. Stimulation signals may be defined or described in accordance with stimulation signal parameters including pulse amplitude, pulse frequency, duty cycle, stimulation signal duration, and/or other parameters. Electrical or magnetic stimulation signals applied to a population of neurons can depolarize neurons within the population toward their threshold potentials. Depending upon stimulation signal parameters, this depolarization can cause neurons to generate or fire action potentials. Neural stimulation that elicits or induces action potentials in a functionally significant proportion of the neural population to which the stimulation is applied is referred to as supra-threshold stimulation; neural stimulation that fails to elicit action potentials in a functionally significant proportion of the neural population is defined as sub-threshold stimulation. In general, supra-threshold stimulation of a neural population triggers or activates one or more functions associated with the neural population, but sub-threshold stimulation by itself does not trigger or activate such functions. Supra-threshold neural stimulation can induce various types of measurable or monitorable responses in a patient. For example, supra-threshold stimulation applied to a patient's motor cortex can induce muscle fiber contractions in an associated part of the body.
Although electrical or magnetic stimulation of neural tissue may be directed toward producing an intended type of therapeutic, rehabilitative, or restorative neural activity, such stimulation may result in collateral neural activity. In particular, neural stimulation delivered beyond a certain intensity, period of time, level, or amplitude can give rise to seizure activity and/or other types of collateral activity. It will be appreciated that certain types of collateral neural activity may be undesirable and/or inconvenient in a neural stimulation situation.
Another concern that arises in association with stimulating a surface site on a patient's cortex is conservation or minimization of applied power while operating a stimulation device. Various types of systems have an implanted pulse generator (“IPG”) and an electrode assembly. The electrode assembly generally has a plurality of contacts that are carried by a common support member, such that the contacts are positionally fixed in close or generally close proximity relative to each other. In operation, the IPG delivers an electrical waveform to the electrode assembly, such that a first set of contacts provides a current delivery path and a second set of contacts provides a current return path. Thus, at any given time during waveform delivery, at least one contact has a positive bias and at least one contact has a negative bias, resulting in the generation of a bipolar field at the surface of the cortex within the area of the stimulation site. The bipolar field has a lower current density in the deeper layers of the cortex compared to the current density at the surface layers, and the bipolar field runs generally parallel to the cranium of the patient in the deeper layers of the cortex. Systems that generate a bipolar field at the stimulation site may require relatively high current levels to achieve an intended or desired therapeutic effect. This may result in increased power consumption, and possibly increase the likelihood of inducing collateral neural activity.